Psychology 2020A/B Chapter Notes - Chapter 3: Drug Metabolism, Vomiting, Aspirin
22 May 2018
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Psychology 2020
Chapter 3
How Drugs Enter the Body
- Goal: drug to be absorbed into the bloodstream
- For psychoactive drugs, a drug effect depends on the drug reaching the brain
- 4 ways drugs are delivered to the blood:
1. Oral Administration
- ingesting a drug by mouth, ingesting it, and absorbing it into the blood through the GI tract
- provides a degree of safety
o poisons taste so bad we spit them out before swallowing
o others cause nausea that we vomit it out
o takes 5-30 mins after ingestion for most of the absorption process to be completed so
there is some time to induce vomiting or pumping of the stomach
o GI tract also contains natural barriers that slow down or prevent absorption of certain
drugs
▪ Drugs pH value
• The interior of stomach is highly acidic so the fate of the drug depends
on how it reacts with that environment
• Weakly acidic drugs (Ex: aspirin) are absorbed better in stomach than
highly alkaline drugs (ex: morphine, heroin, cocaine)
• Ex: insulin is destroyed by stomach acid so it cant be taken orally
• Neutral substance (ex: alcohol) is readily absorbed at all points in GI
tract
• If the drug stays in a nonionized form, it passes more easily into the
blood
• Ionization is determined by the pH of the surrounding environment
• Acidic drug will become ionized in alkaline environment and vice versa
▪ Membrane separating GI tract from capillaries
• Made of 2 layer so fat molecules
• So the substances must be lipid soluble in order to pass through
▪ Liver
• If it enters the blood (either through the stomach or small intestine), it
must pass through the liver for a screening process before being
released into general circulation
• Enzymes in liver can break down (metabolize) the molecular structure
of certain drugs→reducing the amount that eventually reaches the
blood
• This is called first pass metabolism →plays an important role in
protecting us from potentially toxic substances that might have been
ingested by mouth
▪ Blood brain barrier
- So because of these natural barriers, orally administered drugs must be taken at higher doses bc
some of the drug will not make it through to the blood
- Internal state of GI tract changes constantly
- There are things that will change, making it more or less likely that a drug will reach the
circulatory system:
o Presence or absence of undigested food
o Whether the drug interacts with the undigested food
o Activity level of specific liver enzymes
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2. Injection
- Risk of bacterial or viral infection if needle is not sterile
i. Intravenous (into a vein) injection
o directly into the blood through a hypodermic syringe and needle
o Very fast and intense effects
▪ Ex: heroin arrives at the brain in 15s
o In medicine, intravenous injections provide a lot of control over dosage and the
opportunity to administer multiple drugs at the same time
o Disadvantage: effects are irreversible
▪ There is no turning back if a mistake is made →Unless an antidote (another
drug that can counteract the first one)
▪ Repeated injections through a particular vein can cause the vein to collapse or
develop a blood clot
ii. Intramuscular injections
o Delivered into a large muscle and is absorbed into the blood through capillaries
o Slower absorption times than intravenous but they can be administered rapidly in
emergency situations
o Ex: vaccines
iii. Subcutaneous injections
o Needle is inserted into the tissue just underneath the skin
o Skin has way less blood supply relative to the muscle
o Slowest absorption time
o Used when we want precise control over the dosage and a stead absorption into the
blood
o Skin may be irritated so only small amounts of drug can be injected this way
o When involved in drug abuse, these injections are called skin popping
3. Inhalation
- Inhalation in some form of gas or vaporous state
- Our body has an extremely efficient system for getting O2 to its destination
- Psychoactive effect of an inhaled drug is even faster than a drug delivered through intravenous
injection
o About 5-8s for it to travel from lung to brain
- Burn it and breather in the smoke borne particles
o nicotine form cigarettes
o opium
o TCH from weed
o free based cocaine
o crystal form of methamphetamine
- can control the dosage precisely
- disadvantage: long term hazards of breathing particles in the air that contain not only the active
drug but also tars and other substances by the burning process
o emphysema
o asthma
o lung cancer
o linings leading from the throat to the lungs will be severely irritated over time
4. Absorption through Skin or Membranes
i. Intranasal – into the nasal cavity
o Once in the nose, it adheres to thin mucus membranes and dissolves through the
membranes into the bloodstream
o Snuff tobacco
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o Cocaine
ii. Sublingual – applied under the tongue
o Chewing tobacco
o Cocaine containing coca leaves
o Nicotine chewing gums
o Nitroglycerin tablets
iii. Rectum
o Medicines can be inserted in the rectum
o Gradually melts and is absorbed through the thin rectal membranes into the blood
o Less reliable than oral administration
o May be necessary if the person is vomiting or unconscious
iv. Transdermal patch – device attached to skin that slowly delivers the drug through the skin
o Long term administration of nitroglycerin estrogen, nicotine, medications for motion
sickness, parkinsons, and alzheimers
o Insulin
How Drugs are Inactivated and Excreted
- Elimination half life - the length of time it takes for a drug to be reduced to 50% of its
equilibrium level in the bloodstream
i. Excretion in the urine after a series of actions in the liver and kidney
o Most common way
o Biotransformation – process of changing the molecular structure of a drug into forms
that make it easier for the body to excrete it
o Products of biotransformation are metabolites
o Goal of metabolism is to make the drugs water soluble so they are passed along to the
kidneys and eventually excreted in the urine
o If they are less water soluble, they are reabsorbed into the intestines and excreted
through defecation
o Factors that influence biotransformation process and urinary excretion and in turn the
rate of elimination from the body:
▪ Larger quantity of drug, the faster the body tries to get rid of it
• EXCEPTION: alcohol (the rate of biotransformation is constant)
▪ Activity of enzymes required for biotransformation
• Cytochrome P450 (family of liver enzymes) related to drug metabolism
• Some drugs can act on these enzymes to reduce or increase their
activity
• Enzyme induction – when repeated drug taking results in greater
production of liver enzymes that metabolize the drug and similar drugs
• Enzyme inhibition – when repeated drug taking stops the action of
specific liver enzymes and metabolism of the drug slows down
• Drug competition – drugs taken in combination occupy all of the
available liver enzymes, leaving the excess drugs nonmetablized and
resulting in dangerously toxic levels
• age of an individual →enzyme activity in the liver decline after 40 yrs
(older people eliminate drugs more slowly)
▪ chemical properties of the drug
• fat soluble drugs are eliminated more slowly than water soluble drugs
ii. Exhaled breath
iii. Feces
iv. Sweat
v. Saliva
vi. Breast milk (in the case of nursing mothers)
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